1. Field of the Invention
This invention relates to a living subject adaptive blood flow control system and more particularly to a plasmapheresis blood flow control system which optimizes blood flow by limiting the blood flow rate in accordance with a flow control curve determined individually for each subject from actual subject data.
2. Discussion of the Prior Art
Plasmapheresis systems as well as other systems are known which require the extraction or reinfusion of bodily fluids from or to a living subject. The subject is typically a living human or animal subject but might also be a cadaver. In the case of a plasmapheresis system whole blood is extracted from the subject, plasma is separated from the whole blood, and an extraction product containing a higher concentration of blood cells than the whole blood is reinfused back to the subject while the separated plasma is retained and used for desired purposes. Frequently, a selected volume of saline solution is infused into the donor to replace the volume of plasma separated from the whole blood.
To optimize utilization of processing equipment and support personnel and minimize inconvenience and discomfort to the subject, it is desirable to remove or reinfuse bodily fluids as rapidly as possible. However, physiological restrictions on flow rates impose practical limitations on how fast pumping can proceed.
During extraction, if the pumping rate exceeds the flow capacity of a vein into which a phlebotomy needle is inserted, the intravenous pressure will drop below atmospheric pressure and the vein sidewalls will collapse under atmospheric pressure. When this collapsing vein problem occurs the blood pump must be stopped or significantly slowed until intravenous blood flow restores the intravenous pressure to a point greater than atmospheric pressure.
Oftentimes when the vein collapses about the needle the end of the needle will become occluded against the sidewall of the vein. When this happens the needle will frequently become embedded within the vein sidewall or will be sealed to the vein wall by virtue of the negative pressure within the needle and tubing that can be developed following a sudden occlusion. The needle then remains occluded even after the blood pump is fully stopped. It may even become necessary to remove and reposition the needle at the expense of considerable additional time delay.
Reinfusion presents a somewhat different flow rate problem from extraction. During reinfusion, if the pumping flow rate exceeds the vein flow capacity, the intravenous pressure increases until either the phlebotomy needle is forced out of the vein or the vein swells or even bursts or leaks into surrounding tissue. This creates an undesirable hematoma.
Predicting the optimum flow rate is difficult because it varies considerably from subject to subject. Even for a given subject the flow rate capacity can vary considerably over a period of time depending upon how vigorously flow stimulating exercises such as hand squeezing are being performed.
Attempting to optimize the blood flow rate by sensing flow path pressure adjacent the needle is uncertain because the pressure drop across the needle varies substantially with flow rate, hematocrit dependent blood viscosity and needle size parameters. It is therefore common to rely on a gravity driven flow rate far below the optimum or a pumping rate that is known to be well within the blood flow capacity of most subjects. This may be far below the optimum flow rate.
One arrangement in which a plasmapheresis system serves as a reservoir for receiving and returning bodily fluids is described in U.S. Pat. No. 4,086,924 to Latham, Jr. for "Plasmapheresis Apparatus". In this system extraction occurs under vein pressure and gravity. A multi-rate blood pump for the plasmapheresis system is accelerated or decelerated to match this flow rate. Reinfusion occurs at a predetermined rate with the blood pump set to a relatively low speed condition.